1. Field of the Invention
The present invention relates to an organic EL (Electroluminescence) element, and also to a method of producing it.
2. Description of the Related Art
An organic EL element is a surface emitting device configured so that electrodes of a unit area are opposingly placed above a substrate, one of the electrodes is used as an anode to which a positive voltage is to be applied, the other electrode is used as a cathode to which a negative voltage is to be applied, and organic material layers including a luminescent layer are interposed between the electrodes. When a voltage is applied between the electrodes, electrons are injected from the cathode into the luminescent layer, holes are injected from the anode into the luminescent layer, and electron-hole recombination occurs in the luminescent layer, thereby causing luminescence. When a plurality of such organic EL elements are formed in a matrix as unit surface emitting elements above a substrate and then driven in the dot-matrix system, it is possible to form a self-luminescent flat panel display which can display a high-definition image.
Such an organic EL element is characterized in that the luminescent layer is formed by an organic material. In order to inject electrons or holes and improve the efficiency of electron-hole recombination, usually, a hole injecting layer, a hole transporting layer, an electron injecting layer, and an electron transporting layer are selectively placed between the luminescent layer and the electrodes, and each of the layers is formed by a thin layer of about 30 to 200 nm. As described above, organic material layers of an organic EL element are configured by a stack of thin films. When a foreign matter enters the organic material layers during a process of growing the layers, or when the electrodes are rugged, therefore, local ungrown portions are easily produced during a growing step. As a result, a structure is formed in which such ungrown portions tend to cause a partial short circuit between the electrodes, abnormal emission and dark spot formation due to occurrence of a leakage current, and the like.
As a countermeasure for the above, a technique is disclosed in, for example, JP-A-2001-68272 and JP-A-2000-91067. In the technique, a hole injecting and transporting material is heat-treated above an anode (hole injecting electrode) at a temperature that is equal to or higher than the glass transition temperature of the material, and above-mentioned ungrown portions are covered by using a phenomenon in which the heat-fused material merges with neighboring materials.
In the conventional technique, one layer which is formed above an anode and made of a hole injecting and transporting material is heat-treated to cover local ungrown portions. Usually, a foreign matter such as dust, or ruggedness which causes the above-mentioned troubles is considerably large as compared with the film thickness of each organic material layer. Even when ungrown portions of one layer to be heat-treated may be covered by a heating process of the layer, therefore, there is the possibility that ungrown portions may be formed in another layer which is stacked on the foreign matter or the ruggedness. In the case where ungrown portions are formed in another layer, there arise the problems of such abnormal emission and dark spot formation due to occurrence of a leakage current, in the same manner as the case which has been described above.
In plural organic material layers to be stacked, different materials are used depending on the functions of the layers. However, these materials do not always have a satisfactory junction property of an interface. Depending on the selected materials, therefore, charges cannot smoothly traverse an interface, and excellent voltage-current characteristics cannot be obtained.
By contrast, in the case where an organic material layer formed above an electrode is heat-treated, slight denaturation occurs in the surface state. When another organic material layer is stacked above the layer, therefore, there is the possibility that the junction property of the interface between the stacked organic material layers is impaired and traversing of electrons or holes through the interface is hindered. In this case also, there is the possibility that voltage-current characteristics of a resulting organic EL element are adversely affected and luminance characteristics are degraded.